1. Field of the Invention
This invention relates generally to the field of auxiliary equipment for marine seismic cables and, more particularly to the attachment of equipment to cables, and even more particularly to the shape and construction of the attachment point.
2. Description of the Prior Art
In the course of conducting marine seismic surveys for oil and gas, it is customary to tow cables, often 3 to 10 kilometers in length, beneath the surface and astern of a survey ship. As many as twelve of these cables are towed at one time. Each cable contains multiple hydrophones as well as associated electronics and other sensors required in the data acquisition process. The cables are called “streamers” and are constructed using semi-solid plastic material or oil filled flexible tubing. They are typically two to three inches in diameter.
When conducting seismic surveys, the depth of the cable must be maintained at a precise distance from the surface. This is accomplished by the use of depth control devices that are attached to the exterior of the streamer cable using collars which are secured around the cable and result in a bearing comprising an inner race and an outer race that allows the cable to be free to rotate within the attachment point. The outer race and inner race are constructed and assembled so that they can rotate with respect to each other. The outer race is attached to depth control devices. Other types of modules for other purposes are attached in the same manner.
When streamers are retrieved, they are wound on large powered reels and are under tension forces of as high as seven thousand pounds. As the cable comes aboard, all external devices are removed prior to storage on the reel. Removal is accomplished by first unlatching the outer portion of the collar with the inner race of the bearing device attached to the cable. If the cable is wound on the reel with the inner race still attached to the cable, the inner race can cause damage to underlying layers of cables due to conventional shapes and materials used in their construction.
The marine seismic industry uses a prior art inner race that includes two semi-cylindrical halves which are first placed about the streamer cable and then fastened together by screws. After the inner race has been exposed to the salt water of the sea for any time, corrosion can occur in the screwed connection making it difficult if not impossible to remove the inner race. Accordingly, it is usually desired to remove the inner race from the cable to prevent damage to the cable during reeling in. Furthermore removing such screws in order to remove the inner race from the streamer cable is difficult in harsh weather conditions.
The prior art collars are described in a number of patents such as:
U.S. Pat. No. 4,290,124 Cole, Jimmy R. Remote Control Cable Depth Control Apparatus
U.S. Pat. No. 4,709,355 Woods et al Controller for Marine Seismic Activity
U.S. Pat. No. 5,529,011 Williams, Oneil J. Connector for Underwater Cables
The Williams patent mentioned above illustrates a prior art connector 30 as show in FIGS. 1 and 2 attached hereto used to connect one or more pieces of equipment such as a depth control mechanism on a float tube to an underwater streamer cable 10. The cable can be towed underwater. In practice, a plurality of depth control mechanisms and other equipment, such as acoustic transceivers are connected to the cable 10 at intervals along its length, which is sometimes up to several kilometers.
Each connector 30 has generally the shape of a cylinder surrounding the cable 10. The connector 30 includes a cylindrical inner collar, referred to as an inner race 31, which is clamped to the outer surface of the cable 10, and a cylindrical outer collar, referred to as an outer race 40 which surrounds this inner race 31 and can freely rotate about the inner race 31. Connecting members, called pylons (not shown), are connected to one of the outer races 40 but not to the inner races 31. As a result, when the cable 10 is being towed through the water, a depth control mechanism can hang beneath the cable 10 and a float tube can float above the cable without being affected by twisting of the cable 10.
As shown in FIGS. 1 and 2, a prior art inner race includes a plurality of arcuate sections which are combined to define a generally tubular shape with a cylindrical bore for receiving the cable 10. The prior art inner race of FIGS. 1 and 2 comprises two semi cylindrical section 32 fastened together by screws 33. The inner race 31 is connected to the cable 10 so as not to slide along the length of the cable 10 during use. The inner race 31 can be connected to the outer surface of the cable 10, or taped or another member may be attached to the cable 10 near the ends of this inner race 31 to define stops which prevent the inner race 31 from moving longitudinally along the cable 10 when subjected to drag forces. Alternatively, the inner diameter of the inner race 31 may be selected so that when the two sections 32 are combined, the cable 10 is tightly clamped between the sections 32 without the cable 10 being damaged.
On its outer periphery, the inner race 31 has a cylindrical bearing surface 34 for rotatably supporting the outer race 40. Two cylindrical flanges 35 each having a larger outer diameter than the bearing surface 34 are formed at the longitudinal ends of the bearing surface 34. The flanges 35 define stops which limit the axial movement of the outer race 40 when it is mounted on the inner race 31. While the flanges 35 need not have any specific shape, preferably they have no sharp corners on their outer peripheries which could catch against objects underwater.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a screwless arrangement for securing two semi-cylindrical halves of an inner race together about a streamer cable.